The present invention relates to a method of recovering enriched gaseous oxygen from a crude gas containing oxygen by pressure swing adsorption (PSA process).
Oxygen-rich gas, i.e. gas with a high oxygen concentration, obtained by a PSA process is widely utilized for technical fields which require continuous supply of oxygen, i.e. for electric steel making, water treatment by oxygen aeration, pulp bleaching or ozonizers for example. Recently, also in the technical field of combustion, resort is made to combustion in oxygen-rich gas instead of combustion in the air for melting furnace residues, for providing a lower NOx emission or for enhancing efficiency of chemical reaction for example. Moreover, oxygen-rich gas is widely utilized also in the field of biochemistry such as fermentation.
A typical PSA process is a multi-tower PSA process which utilizes an apparatus provided with at least two adsorption towers. In the multi-tower PSA process, the process steps of adsorption, desorption and pressurization are repeated in each of the adsorption towers. These process steps are performed in the adsorption towers at different timings from each other. Various attempts have been made for the improvement of such a multi-tower PSA process and an apparatus used therefor. For example, JP-A-8-239204 discloses a process in which the pressure in an adsorption tower in which adsorption is finished is utilized for pressuring another adsorption tower.
On the other hand, a single tower PSA process which utilizes an apparatus provided with a single adsorption tower is also known as a process for realizing size-reduction, simplification of the apparatus and initial cost reduction. Various attempts have been made also for the improvement of such a single tower PSA process and an apparatus used therefor with respect to the amount and purity of oxygen gas obtained as a product. For example, JP-A-9-29044 discloses a process in which the gas remaining in the adsorption tower upon finishing the adsorption is recovered in a separately provided recovery tank and is returned to the adsorption tower when the desorption is finished for washing the adsorption tower.
However, with the processes disclosed in the gazettes or with other prior art single tower PSA processes, the recovery of oxygen-rich gas is insufficient and there is still room for improvement.
An object of the present invention, which is conceived under the circumstances described above, is to enhance the recovery of oxygen-rich gas in obtaining oxygen-rich gas by a single tower PSA process.
According to a first aspect of the present invention, there is provided a process for recovering oxygen-rich gas by enriching gaseous oxygen contained in crude gas by a single tower pressure swing adsorption which utilizes a single adsorption tower loaded with an adsorbent. In this process, a cycle is repeated which includes an adsorption step for adsorbing an unnecessary component contained in the crude gas by the adsorbent by introducing the crude gas into the adsorption tower to output oxygen-rich gas from the adsorption tower, a desorption step for desorbing the unnecessary component from the adsorbent by depressurizing the adsorption tower, a washing step for introducing washing gas into the adsorption tower to discharge remaining gas in the adsorption tower, and a pressurization step for raising the internal pressure of the adsorption tower. The desorbing step includes recovering semi-enriched oxygen gas existing in the adsorption tower after the adsorption is finished for retention in a recovery tank. The washing step includes introducing part of the semi-enriched oxygen gas retained in the recovery tank into the adsorption tower as the washing gas while discharging the remaining gas from the adsorption tower. The pressurizing step includes raising the internal pressure of the adsorption tower by introducing the rest of the semi-enriched oxygen gas retained in the recovery tank into the adsorption tower.
Preferably, the adsorption tower has a crude gas inlet and a product gas outlet. In the desorption step, the semi-enriched oxygen gas is recovered into the recovery tank through the product gas outlet, whereas the unnecessary gaseous component desorbed from the adsorbent is discharged through the crude gas inlet.
Preferably, the washing step includes introducing part of the oxygen-rich gas into the adsorption tower as the washing gas while discharging the remaining gas from the adsorption tower.
Preferably, the division ratio between the amount of the semi-enriched oxygen gas to be introduced in the adsorption tower in the washing step and the amount of the semi-enriched oxygen gas to be introduced in the adsorption tower in the pressurization step lies in the range of from 65:35 to 97:3 as calculated on the basis of standard state volume. More preferably, the division ratio lies in the range of from 75:25 to 93:7 as calculated on the basis of standard state volume.
According to a second aspect of the present invention, there is provided another process for recovering oxygen-rich gas by enriching gaseous oxygen contained in crude gas by a single tower pressure swing adsorption which utilizes a single adsorption tower loaded with an adsorbent. In this process, a cycle is repeated which includes an adsorption step for adsorbing an unnecessary component contained in the crude gas by the adsorbent by introducing the crude gas into the adsorption tower for outputting oxygen-rich gas from the adsorption tower, a first desorption step for desorbing the unnecessary component from the adsorbent by depressurizing the adsorption tower for discharging the unnecessary component from the adsorption tower while recovering semi-enriched oxygen gas existing in the adsorption tower for retention in a recovery tank after the adsorption is finished, a second desorption step for desorbing the unnecessary component from the adsorbent out of the adsorption tower by depressurizing the adsorption tower without recovering the semi-enriched oxygen gas, a first washing step for introducing washing gas into the adsorption tower while discharging remaining gas from the adsorption tower, a second washing step for introducing part of the semi-enriched oxygen gas retained in the recovery tank into the adsorption tower while discharging the remaining gas from the adsorption tower, and a pressurization step for raising the internal pressure of the adsorption tower by introducing the rest of the semi-enriched oxygen gas retained in the recovery tank into the adsorption tower.
In the process for recovering oxygen-rich gas according to the present invention, the semi-enriched oxygen gas existing in the adsorption tower after the finishing of adsorption is recovered for utilization both for the washing and the pressurization of the adsorption tower. The inventors have confirmed that such a process enhances the final recovery of the oxygen-rich gas as compared with the case where the recovered semi-enriched oxygen gas is utilized solely for the washing of the adsorption tower or solely for the pressurization of the adsorption tower.
The inventors have confirmed that a high recovery is obtained in the case where the division ratio between the amount of the semi-enriched oxygen gas to be introduced in the adsorption tower in the washing step and the amount of the semi-enriched oxygen gas to be introduced in the adsorption tower in the pressurization step lies in the range of from 65:35 to 97:3, and preferably from 75:25 to 93:7 as calculated on the basis of standard state volume.
Other features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings.